As I'm sure you'll know I've been indexing our cemetery. Here's one example of what I've had to put up with. (Names Changed) Michael O'Conner on 1939 register says born 1904. He marries in 1929 using the name Michael O'Conner. He dies in 1981 and the birth date given by his son is 'about 1901' His name is given as Michael Joseph O'Conner and he is 80. I searched for the birth and couldn't find it. I asked his son if he knew what Michael's mothers maiden name was, and he knew the location of his birth. His mother's maiden name was Roberts. Found his birth in 1904 under the name Michael O'Conner mmn Roberts in the correct town. I reported back to the living relative, and said how was it that he had given the birth date of 'about 1901' he said he didn't actually know when his father was born I then asked about the 2nd name of 'Joseph' and explained what I had found. He said that he heard his father answer anyone who asked him his name as 'Michael Joseph O'Conner' so that's what he thought his name was
The old adage proves true - the information is only as good as the knowledge of the person who gave it but quite extraordinary a son did not know his father's dob
I've happened upon several persons who 'after baptism records' have added another name to their Christian name. Makes searches a bit messy. Perhaps this chap was given Joseph at baptism or just added it himself for his own amusement later in life.
My Great Aunt, protestant married a catholic in a catholic church in Liverpool and she had added Veronica as a middle name on the register. Her granddaughter told me when she converted just before she married she chose Saint Veronica.